Displacement measuring instrument

ABSTRACT

A displacement measuring instrument having means for producing a carrier signal, a transducer supplied with the carrier signal from the above means to phase modulate the carrier signal in accordance with a displacement, means for multiplying the phasemodulated signal, means for demodulating the multiplied, phasemodulated signal and means for counting the displacement with the demodulated signal.

United States Patent Inventor Saburo Uemura Kanagawa-lten, Japan Appl.No. 862,413 Filed Sept. 30, 1969 Patented Dec. 7, 1971 Assignee SonyCorporation Tokyo, Japan Priority Oct. 2, 1968 Japan 43/7 1660DISPLACEMENT MEASURING INSTRUMENT 7 Claims, 11 Drawing Figs.

U.S. Cl ..340/l"74.l-B, 324/34 D Int. Cl. G 1 lb 5/04, GOlr 33/00 FieldoiSearch 340/l74.l,

[56] References Cited UNITED STATES PATENTS 2,947,929 8/1960 Bower324/172 3,308,449 3/l967 Uemura 340/l74.l F

Primary Examiner-James W. Moffitt Assistant Examiner-Vincent P. CanneyAllomeys- Lewis H. Eslinger, Alvin Sinderbrand and Curtis,

Morris & Safford ABSTRACT: A displacement measuring instrument having:means for producing a carrier signal. a transducer supplied with thecarrier signal from the above means to phase modulate the carrier signalin accordance with a displacement, means for multiplying thephase-modulated signal, means for demodulating the multiplied,phase-modulated signal and means for counting the displacement with thedemodulated signal.

PATENTED nu: 7 I97! SHEET 1 [IF 3 INVIL'N'IliR. SABURO UEMURA PATENTEDDEB 7197i SHEET 2 OF 3 SABURO L J'EMJRA liY DISPLACEMENT MEASURINGINSTRUMENT CROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to adisplacement measuring instrument and in particular to a measuringinstrument having an interpolating means.

2. Description of the Prior Art There has heretofore been proposed adisplacement measuring instrument for electrically measuringa length ofangle which employs a magnetic scale having reference divisions,commonly called magnetic gratings, formed by recording rectangular orsinusoidal signals of a certain wavelength on a magnetic medium such as,for example, as a magnetic tape or a disc and magnetic heads disposed inopposing relation to the magnetic scale and adapted for moving relativethereto and in which the linear or angular displacement between themagnetic scale and the magnetic heads is converted into an electricsignal to measure a length or angle. At present, interpolation readoutof the order of 10,5 or 2 microns is achieved by the use of a magneticscale having magnetic gratings of a wavelength of about 200 microns anda digital phase difference detector. T he interpolation with the digitalphase differe ce detector necessitates two counters, one providing anincremental indication every wavelength and the other a digitalindication of a value less than the wavelength, that is, theinterpolated value. However, the necessity of providing two countersintroduces complexity into the construction of the counter circuit; andincreases the manufacturing cost of the displacement measuringinstrument. Further, it is difficult to set to zero or null the counterfor digital indication of the interpolated value.

SUMMARY OF THE INVENTION The present invention deals with a displacementmeasuring instrument which includes a transducer supplied with a carrierfrom an oscillator to phase modulate the carrier in accordance with adisplacement, means for multiplying the output of the transducer andmeans for demodulating the multiplied phasemodulated signal and in whicha displacement of one wavelength of the transducer is therebysubdivided.

Accordingly, one object of this invention is to provide a noveldisplacement measuring instrument.

Another object of this invention is to provide an improved displacementmeasuring instrument which is capable of reading out a displacement of avalue less than that of a transducer of one wavelength.

Another object of this invention is to provide a displacement measuringinstrument which indicates subdivided counting pulses in an incrementalmanner.

Another object of this invention is to provide a displacement measuringinstrument which can be easily set at zero.

Still another object of this invention is to provide a displacementmeasuring instrument which is inexpensive and easy to handle.

Other objects, features and advantages of this invention will becomeapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing oneexample of a displacement measuring instrument produced according tothis invention;

FIG. 2 is a connection diagram of a doubler which is suitable for use inthe present invention;

FIGS. 3A to 3E are a series of waveform diagrams for explaining thephase detection and interpolation of a multiplied signal in aninstrument according to the present invention;

FIG. 4 is a block diagram, similar to FIG. 1, showing another example ofthis invention;

FIG. 5 is a connection diagram illustrating one example of a phasedemodulating device which is suitable for use in the instrument of FIG.4 according to the present invention; and

FIG. 6 is a graph showing the output waveform of the device exemplifiedin FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 reference numeral 1indicates a carrier frequently oscillator operating at a certain highfrequency f, 2 a pulser or pulse generator for converting the signalfrom the oscillator 1 into a pulse.

The instrument shown on FIG. I further comprises a frequencydemultiplier circuit 3 (3,,,-3,,) for producing a signal having afrequency of s" of the carrier frequency f (n being a desired positiveinteger), that is, a frequency of f/Z", a transducer 4 for convertingits displacement into a phase shift of the demultiplied signal havingthe frequency of 172', a frequency doubler circuit 5 (5,,,-5,,-,) formultiplying the resulting phase-modulated signal back to the carrierfrequency and a detector circuit 6 for detecting the phase of theresulting multiplied signal with reference to the carrier frequency,thereby producing a phase-modulated signal having a phase shift N timesthe displacement of the transducer 4.

In the illustrated example a signal of the frequency f derived from theoscillator l is fed through the pulser 2 to the frequency demultipliercircuit 3 which may be, for example, a flip-flop circuit FF (of fivestages in this case) to be demultiplied to W and the resulting signal isapplied as a carrier through an amplifier 7 to magnetic heads H and Hmaking up the transducer 4, thus producing phase-modulated carriers of afrequency of 172"" at the output side of the magnetic heads. Thephase-modulated carriers are frequency-multiplied 2"" times by thefrequency doubler 5 (of four stages) to raise its frequency back up to fand is then converted into a pulse by a pulser 8. Then, the multipliedfrequency is phase detected by a phase difference detector 6 withreference to the frequency of the oscillator l. The phase differencedetector may be of a conventional type, for example, one in which aflip-flop circuit FF serving as the detector 6 is set and reset by thetwo signal pulses derived from the aforementioned pulsers 2 and 8,respectively, by which is produced a rectangular wave of a widthproportional to the phase difference between the two signals and theresulting rectangular wave is integrated by a filter 9. The magneticheads H and H making up the transducer 4 are of the modulation type suchas are disclosed in the aforementioned U.S. Pat. No. 3,308,499. Thesemagnetic heads H and H are spaced apart from each other by a distance of)\(n+/4) where A is one wavelength of a magnetic scale M. This is notrelated directly to this invention and hence will not be described indetail. A phase shifter 18, connected between the output side of theamplifier 7 and the magnetic head H shifts the phase of the outputsignals from the amplifier to the heads H and H by The outputs of themagnetic heads H and H are combined together by a mixer circuit 10, theoutput of which is fed to the frequency doubler circuit 5.

A description will hereinbelow be given in connection with the frequencymultiplication of the phase-modulated signal in the instrument of thisinvention above described.

Assume that the displacement measuring instrument having the magneticheads disposed opposite the magnetic scale produces the following outputvoltage:

where w,- is an angular frequency of a carrier, A a wavelength of themagnetic scale and x a relative displacement between e sin 2th! With theprovision stages the frequency doublers (n being a desired positiveinteger), the frequency and phase of the output voltage are bothmultiplied 2" times as follows:

11-1: n-l n-LE) 6 5111 (2a)., t+2 A When the carrier is at a constantfrequency, it can be multiplied N times (N=2, 3, 4, by a usualmultiplier but it is phase-modulated and its instantaneous frequencyvaries about 10 percent of the carrier frequency, so that it isdifficult in practice to greatly multiply it with the single-stagemultiplier. The use of the frequency doubler eliminates the difficultyin such a case.

With reference to FIG. 2 a description will be given of a frequencydoubler circuit usable in the present invention.

Reference numeral 11 indicates a tank circuit of a high frequency j,which serves as a primary side of a transformer T and diodes D, and D,are connected to its secondary side, constituting a full-wave rectifier.The rectified output signal of the rectifier is applied to an amplifyingtransistor T, to derive from its output side an amplified signal of afrequency 2] and the resulting signal is shaped by a resonance circuit12 to produce a sine wave of the frequency 2]. Further, a similarfull-wave rectifier 13 is formed on the secondary side of a transformerT using the resonance circuit 12 as the primary side. Thus, thefrequency is sequentially doubled. Accordingly, the frequency f (f/ l6), to which the frequency f of the highfrequency oscillator 1 has beendemultiplied by the flip-flop circuits of five stages and the transducer4 as shown in FIG. I, can be multiplied up to f by the frequencymultiplier 5 employing the doublers of four stages.

Next, a description will be given of means for interpolating thefrequency-multiplied signal produced as above described.

The example of FIG. 1 employs the phase detection method for theinterpolation of the frequency-multiplied signal. More specifically, inthe case of the phase-modulated signal being multiplied N times, thecarrier frequency is increased N times and the phase shift is 2N1rperone wavelength as expressed by the equation (4). Consequently, theinterpolation of the multiplied signal at the rate of UN in anincremental manner can be achieved by counting a pulse at every phaseshift 21r of the carrier.

The curve appearing in FIG. 3A represents a signal of the oscillator land FIG. 3B shows a signal produced by sequentially shifting the phaseof the above signal. The latter signal is a multiplied andphase-modulated frequency. In such a case, the flip-flop circuit ofdetector 6 is set and reset with the two signals as previously describedwith reference to FIG. 1, thereby producing a rectangular wave such asis shown in FIG. 3C and which is of a width proportional to the phasedifference d between the two signals. This rectangular wave is producedat intervals of Zn and is integrated by the filter 9, producing a rampwave such as is shown in FIG. 3D, and in which the phase difference Ibetween the two signals jumps every 21r. The resulting ramp wave isdifferentiated by a differentiation circuit 14 to provide a pulse suchas is shown in FIG. 3E. Where the displacement is in a negativedirection, the width of the rectangular wave of FIG. 3C decreases, thepolarity of the ramp wave of FIG. 3D is reversed to produce a pulseshown on FIG. 3E which is opposite in sense to that shown on FIG. 35 andthe pulse of FIG. 3E is produced at intervals of A/N. Namely, with theinstrument of FIG. 1, the pulse of FIG. E or E is produced in responseto the positive or negative direction of the displacement and is appliedto a counter 15 to indicate the amount of the displacement.

Accordingly, when the magnetic scale is moved a distance correspondingto one wavelength a phase shifi of 2N1'r( N==2"' is caused, so thatinterpolation of UN can be achieved.

When one wavelength of the magnetic scale is 160 microns, theinterpolation of one-sixteenth requires five stages of frequencydemultipliers and four stage of frequency doublers.

In the foregoing each subdivided wavelength A/N is counted in anincremental manner. Consequently, there are the following advantages:

1. The instrument can be set to read null at any desired interpolatedvalue, and

2. The counter may be a reversible one.

The counting is distinctly separated from the detection for theinterpolation.

Turning now to FIG. 4, another example of this invention will bedescribed in connection with amplitude modulation. In FIG. 4 similarelements to those appearing in FIG. 1 are identified by the samereference numerals. In the illustrated example the phase-modulatedfrequency of the carrier N times frequency-multiplied is demodulatedwith demodulators 6a and 6b to be converted into a low-frequency signal.The il- Iustrated circuit is completed by a 0 phase shifler l6 and a U4interpolator 17. The phase demodulators 6a and 6b may be each in theform of the synchronous detector depicted in FIG. 5.

In the synchronous detector of FIG. 5, reference character e indicates asignal source, diodes Da and Db are connected to both ends of asecondary coil of a transformer T to achieve full-wave rectification andresistors R and R, are connected in series to the output ends of thediodes Da and Db to apply the output of a carrier source e, between theconnection point P, of the resistors and the center tap of the secondarycoil of the transformer T through a secondary coil of a transformer TReference character F designates a filter, though which is produced anoutput signal of the demodulator at terminals r. If signals to beapplied to the two detectors or demodulators 6a and 6b are displaced1r/2 apart from each other, there are derived from the output terminalsof the demodulators lowfrequency signals which are phased apart fromeach other, as clearly shown in FIG. 6. Such output signals are asfollows:

E ea -cos N A These output signals are exactly the same as thoseobtained by detecting the outputs of the magnetic heads with thesynchronous detectors and the wavelength A is replaced with A/N. Theabove output signals are fed to M4 interpolated 17 to be converted intopulses in both directions, which are thereafier indicated by a counter15. For the interpolation of the unit of 10 microns in the case of usinga magnetic scale having one wavelength of microns, it is sufficient toemploy two stages of frequency doublers and to multiply the carrier fourtimes.

With the above interpolation method, the precision of the interpolationis determined when the outputs of the magnetic heads are combinedtogether to provide the phase-modulated signal. The phase shift of thefrequency doubler circuit has no influence upon the precision.

An error of 90 in phase of the carrier frequency in the demodulator inan error of the M4 interpolator, so that a difference of :10" does notmatter for the overall error in interpolation. The precision of theinterpolation may be considered to be dependent only upon the portionwhere the outputs of the magnetic heads are combined together to providethe phasemodulated signal. in the interpolation of M16 the error can beheld to for example, one-fourth to one-sixth of the counting unit. Thevelocity is dependent upon the carrier frequency.

Where the frequency of the oscillator l is 50 kHz., an excitingfrequency of the magnetic heads is 50/8=6.25 kHz. and the outputs of themagnetic heads which have a frequency of 12.5 kHz. are multiplied fourtimes up to 50 kHz., a signal of up to 5 kHz. is tolerable in thedemodulator. The resulting signal is applied to the A14 interpolator, inwhich case a pulse of up to 20 kHz. is permissible. With one pulse beingmicrons, a velocity of cm./sec. (l2m./min.) can be measured.

Even if the carrier frequency is raised two to four times, there is nosubstantial difficulty, so that a rise of the frequency of theoscillator 1 up to 200 kHz. and the exciting frequency of the magneticheads up to kHz. allows ease in the design of the filter and thepermissible velocity can be raised up to 50 cmJsec.

As compared with the interpolation method of the type using the digitalphase difierence detector, the interpolation method according to thepresent invention has the following advantages.

1. Since the interpolator is based on the incremental pulse system, itsconnection to a reversible counter and the design of the circuits arefacilitated.

2. Since the interpolator is based on the incremental system, theinstrument can be set at zero simultaneously with the interpolation.Further, the instrument is easy to handle and eliminates the use of agoniometer circuit, so that the manufacturing cost of the instrument islow.

3. Since the counter and the interpolator can be completely separatedfrom each other, the manufacture and the assembling are facilitated.

4. The circuits are simple and the instrument of this invention isadvantageous in its stability and cost.

It will be apparent that many modifications and variations of theabove-described embodiments may be effected without departing from thescope of the novel concepts of this invention.

I claim as my invention:

1. A displacement measuring instrument comprising oscillator means forproducing a carrier signal, demultiplier means acting on said carriersignal to provide a demultiplied carrier signal, transducer meanssupplied with said demultiplied carrier signal and being operativetophase modulate said demultiplied carrier signal in accordance with adisplacement, means for multiplying the phase-modulated demultiplied carrier signal, means for demodulating the multiplied phasemodulatedsignal, and counting means operated by the demodulated signal toindicate said displacement.

2. A displacement measuring instrument according to claim 1, in whichsaid means for multiplying the phase-modulated signal is a frequencydoubler.

3. A displacement measuring instrument according to claim 1, whereinsaid means for demodulating the multiplied phasemodulated signalincludes phase-comparator means comparing the phases of said multipliedphase-modulated signal and a reference signal.

4. A displacement measuring instrument according to claim 1, in whichsaid means for multiplying the phase-modulated signal brings back thefrequency of the latter to the frequency of said carrier signal, andsaid reference signal is the carrier signal from said oscillator.

5. A displacement measuring instrument according to claim 1, in whichsaid means for demodulating the multiplied phasemodulated signalincludes two amplitude detectors.

6. A displacement measuring instrument according to claim 5, in whichsaid amplitude detectors are supplied with said multipliedphase-modulated signal and with reference signals that are respectivelyout of phase with each other.

7. A displacement measuring instrument according to claim 6, in whichsaid reference signals are obtained from said oscillator means.

t i I! i t

1. A displacement measuring instrument comprising oscillator means forproducing a carrier signal, demultiplier means acting on said carriersignal to provide a demultiplied carrier signal, transducer meanssupplied with said demultiplied carrier signal and being operative tophase modulate said demultiplied carrier signal in accordance with adisplacement, means for multiplying the phase-modulated demultipliedcarrier signal, means for demodulating the multiplied phase-modulatedsignal, and counting means operated by the demodulated signal toindicate said displacement.
 2. A displacement measuring instrumentaccording to claim 1, in which said means for multiplying thephase-modulated signal is a frequency doubler.
 3. A displacementmeasuring instrument according to claim 1, wherein said means fordemodulating the multiplied phase-modulated signal includesphase-comparator means comparing the phases of said multipliedphase-modulated signal and a reference signal.
 4. A displacementmeasuring instrument according to claim 1, in which said means formultiplying the phase-modulated signal brings back the frequency of thelatter to the frequency of said carrier signal, and said referencesignal is the carrier signal from said oscillator.
 5. A displacementmeasuring instrument according to claim 1, in which said means fordemodulating the multiplied phase-modulated signal includes twoamplitude detectors.
 6. A displacement measuring instrument according toclaim 5, in which said amplitude detectors are supplied with saidmultiplied phase-modulated signal and with reference signals that arerespectively 90* out of phase with each other.
 7. A displacementmeasuring instrument according to claim 6, in which said referencesignals are obtained from said oscillator means.